1. Field of the Invention
The present invention relates in general to inflators for airbag supplemental restraint systems. In particular, the present invention relates to an improved inflator having a filter of reduced size, simplified gas generant loading through the use of cartridges, and adaptive capability.
2. Description of the Related Art
Airbag supplemental restraint systems are increasingly common in contemporary vehicles. Such systems typically include a collision sensor to detect an impact, an inflator which produces a quantity of gas very rapidly upon receipt of a signal from the sensor, and a flexible cushion which is inflated by this gas. The occupant of the vehicle is restrained by this cushion, which deflates to lessen the forces upon the occupant, thus reducing or eliminating injury.
Various types of inflators are known, which operate upon various principles. Many inflator types include a quantity of pyrotechnic gas generant. This generant is in a solid form, typically pellets, granules, etc., until it reaches a high temperature, at which point the generant converts to a gas in an energetic reaction. While the great majority of the generant converts to a high temperature gas, some particulate matter, often referred to a slag, is produced. This slag is also quite hot, and is often entrained in the gas flow from the inflator. If a sufficiently large quantity of this very hot slag entered the cushion, it might impact against a portion of the cushion contacting the occupant, causing burns to the occupant. Similarly, if the gas inflating the cushion were too hot, the heated cushion would burn the occupant. To avoid this, inflators containing pyrotechnic generant have typically included filters to treat the gas. These filters serve the dual purposes of removing slag and cooling the gas.
For passenger side airbags, these inflators have commonly been formed by a tubular metal housing having exit ports extending therethrough, a tubular filter within this housing, and a quantity of gas generant within the tubular filter. In operation the generated gas would of course pass through the filter to reach the exit ports, since the filter is interposed between the generant and the ports. However, this gas flow is not uniform, but is mainly in the areas of the filter opposed to the exit ports. It is in these areas that the majority of slag filtering occurs, although heat transfer, and thus cooling of the gas, occurs about the entire periphery. While this arrangement is serviceable, there is a constant desire to reduce the size, weight and cost of inflators.
One arrangement directed toward these goals is shown in U.S. Pat. No. 5,478,112 to Knobloch. Knobloch shows that the housing may be formed with two longitudinal ribs along the interior, with the exit ports between the ribs. The filter, in the form of resilient mesh layers, may then be formed not as a tube, but as a section of a tube having a limited angular extent. This limited angular extent, or width, is slightly greater than the distance between the ribs, such that the filter may be forced between the ribs. The elastic nature of the metal mesh then secures the filter in place, and reduces the possibility of gas flow around the edges of the filter (i.e., between the filter and the ribs), referred to as flow-by.
This arrangement reduces that amount of filter material required, and thus can reduce the weight of the inflator. Further, and as described more fully below, the area in the housing which was formerly occupied by the removed portion of the filter may instead be filled with the gas generant. As such, the same volume of generant may fit in a shorter length of housing, serving to reduce the size of the inflator.
While this arrangement provides advantages over the prior art, difficulties remain. For example, the filter must have a very tight fit against the ribs to ensure there is no flow-by. This tight fit causes difficulty in assembly, since care must be taken to insert the metal mesh filter in a manner to maintain its resilience, without bending or plastic deformation. The required fit is also problematic over the life of the inflator. While the fit may be initially tight, the tendency of metals to creep may result in a relaxation of the fit in the years the inflator may wait before activation.
Further, the generant of Knobloch is loose within the housing. It is this loose packing which permits the generant to occupy the area of the removed filter portion and realize the reduction in length. The present inventors have found it desirable, however, to pack the generant into cylindrical cartridges, and to then mount these cartridges within the housing. These cartridges of course do not permit flow of the generant about the filter, and as such, less of a length reduction is realized, as is described more fully below. Further, portions of the interior of the housing will be void, without any filter or generant. It is known in the art that such void areas reduce the performance of the inflator, often resulting in the need for more generant or initiating material. This larger volume of generant increases the length of the inflator, so that little or no length reduction would be achieved when using generant cartridges or solid generant blocks.
Another remaining problem is treatment of the gas. The reduced amount of the filter will of course be formed with a sufficient thickness to adequately remove the slag from the gas. However, even such a "thick" filter will have a volume less than that of the original tubular filter. This reduced volume of the filter also results in reduced amounts of cooling capacity. As such, the gas will likely have a higher, and possibly dangerous, temperature.
The present invention avoids these difficulties. The problem of flow-by is eliminated by formation of reduced porosity sections on the edges of the filter, such that the gas will flow through the central portion of the filter. These reduced porosity sections are preferably formed by compressing the edges of the filter to reduce its thickness. This advantageously permits the introduction of a larger generant cartridge, greatly reducing the problem of free, void volume in the inflator. This also greatly reduces the necessary length of the inflator. Finally, the problem of gas treatment is solved by providing the cartridge with a separate pre-cooling screen which acts as a heat sink to reduce gas temperature before it reaches the filter proper. With these features, an inflator may be provided which is smaller and approximately 20% lighter than a prior art inflator with equivalent performance.